A RFeB-based magnet was found by Sagawa et. al in 1982, and has an advantage that many magnetic properties such as residual magnetic flux density are higher than those of permanent magnets in the related art. Accordingly, the RFeB-based magnet has been used in various products such as a drive motor of a hybrid car and an electric car, a motor for electrically-assisted bicycles, an industrial motor, a voice coil motor of a hard disk drive and the like, a high-performance speaker, a headphone, and a permanent magnet-type magnetic resonance diagnostic device.
Early RFeB-based magnets have a defect that among various magnetic properties, a coercive force Hcj is relatively low. However, it has been found that the coercive force is improved by making the heavy rare earth element RH be present inside the RFeB-based magnets. The coercive force is a force that resists inversion of magnetization when a magnetic field in a direction opposite to a direction of the magnetization is applied to a magnet, but it is considered that the heavy rare earth element RH hinders the inversion of magnetization and thus has an effect of increasing the coercive force.
When examining a magnetization inversion phenomenon in the magnet in detail, there is a characteristic that the magnetization inversion occurs at first in the vicinity of a grain boundary of crystal grains and is diffused to the inside of the crystal grains therefrom. Therefore, in a case where the magnetization inversion at the grain boundary is blocked at first, it is effective for prevention of the magnetization inversion of the entirety of the magnet, that is, an increase in the coercive force. Accordingly, the heavy rare earth element RH should be present in the vicinity of the grain boundary of the crystal grains.
On the other hand, when considering the entirety of main phase grains, if an amount of the RH increases, a residual magnetic flux density Br decreases, and thus there is a problem that the maximum energy product (BH)max also decreases. In addition, the RH is a rare resource and is expensive, and a production area is localized, and thus it is not preferable to increase the amount of RH. Accordingly, it is preferable that the RH is present in a small amount at the inside of the crystal grains, and be present in a large amount (unevenly distributed) in the vicinity of a surface (in the vicinity of the grain boundary) to increase the coercive force (to prevent a reverse magnetic domain from being formed as much as possible) while suppressing the amount of RH as much as possible.
As a method of unevenly distributing the RH in the vicinity of the surface rather than the inside of the crystal grains, a grain boundary diffusion method is known (for example, refer to Patent Document 1 and Patent Document 2). In the grain boundary diffusion method, a powder, which contains the RH as an elementary substance, a compound, or an alloy (hereinafter, a powder that contains the RH is referred to as “RH-containing powder” regardless of the type such as the elementary substance, the compound, and the alloy), and the like is attached to a surface of the RFeB-based magnet, and the RFeB-based magnet is heated. According to this, the RH penetrates to the inside of the magnet through the grain boundary of the RFeB-based magnet, and thus atoms of the RH are diffused only in the vicinity of the surface of the crystal grains. Hereinafter, an RFeB-based magnet before performing the grain boundary diffusion process is referred to as a “base material” and is discriminated from an RFeB-based magnet after performing the grain boundary diffusion process.
There are various methods of attaching the RH-containing powder to the base material. Patent Document 1 discloses that the base material is immersed in a turbid solution in which TbF3 powder that is an RH-containing powder and ethanol are mixed, and then the base material is pulled up from the turbid solution and is dried, thereby attaching the RH-containing powder to the surface of the base material. However, in this method, it is difficult to control an amount of the RH-containing powder that is attached to the surface of the base material, and it is also difficult to uniformly attach the RH-containing powder to the surface of the base material in an arbitrary thickness. Therefore, the rare and expensive RH-containing powder is consumed more than necessary.
On the other hand, Patent Document 2 discloses a method of applying (attaching) a mixture obtained by mixing the RH-containing powder and an organic solvent to the surface of the base material by using a screen printing method. Specifically, a plurality of flat plate-shaped base materials are arranged, and a screen in which a plurality of transmission portions capable of transmitting the mixture therethrough are provided in correspondence with the position of the base materials is extended on the surface of the base material. The mixture is supplied on the screen, and then a surface of the screen is scrubbed with a squeegee, thereby attaching the mixture to the surface of the base material through the screen at the transmission portions. Accordingly, it is possible to apply the mixture to the surface of the respective base materials in a uniform thickness, and thus the RH-containing powder is not consumed more than necessary.
In addition, the RFeB-based magnet is largely classified into (i) a sintered magnet obtained by sintering a raw material alloy powder containing a main phase grain as a main component, (ii) a bonded magnet obtained by consolidating raw material alloy powders with a binding agent (binder composed of an organic material such as a polymer and an elastomer) and by molding the consolidated powders, and (iii) a hot-plastic worked magnet obtained by performing a hot press working and hot plastic working with respect to a raw material alloy powder (refer to Non-Patent Document 1). Among these magnets, the grain boundary diffusion process may be performed in (i) sintered magnet and (iii) hot-plastic worked magnet in which the binder of the organic material is not used and thus heating during the grain boundary diffusion process can be performed.    [Patent Document 1] JP-A-2006-303433    [Patent Document 2] WO2011/136223    [Patent Document 3] JP-A-2006-019521    [Non-Patent Document 1] “Development of Dy-omitted Nd—Fe—B-based hot worked magnet by using a rapidly quenched powder as a raw material” written by Hioki Keiko and Hattori Atsushi, Sokeizai, Vol. 52, No. 8, pages 19 to 24, General Incorporation Foundation Sokeizai Center, published on August, 2011.